1. Field of the Invention
The present invention relates to constant-current circuitry suitable for use in regeneration units, such as magnetic disks, optical disks, or magneto-optic disks. More particularly, it relates to constant-current circuitry suitable for use as a semiconductor IC.
2. Related Background Art
Constant-current circuitry used in a semiconductor IC, for example, that shown in FIG. 1A, has hitherto been known.
In FIG. 1A, reference characters T.sub.1 through T.sub.n denote NPN transistors; reference characters T.sub.a and T.sub.b denote NPN transistors for supplying the NPN transistors T.sub.1 through T.sub.n with a constant current; and reference characters R.sub.0 through R.sub.n denote resistors. Numeral 1 denotes a power supply line connected to a power supply; 2 a GND line retained by a ground potential; 3 a constant-current source; and 6 a base line for feeding the constant current to the NPN transistors T.sub.1 through T.sub.n.
The feature of the constant-current circuitry shown in FIG. 1A is that the base line 6 of the NPN transistors T.sub.1 through T.sub.n is commonly utilized and is equally biased in order to feed the constant current to all the NPN transistors T.sub.1 through T.sub.n connected to circuits 1, 2.
The constant-current circuitry shown in FIG. 1A, however, has a technical problem to be eliminated, that is, the circuit block 1 and the circuit block 2 interfere with each other. This is because the base line 6 of the NPN transistors T.sub.1 through T.sub.n is commonly utilized.
To solve such a problem, constant-current circuitry has been known in which the constant-current source of a PNP transistor separates circuits from each other which are not intended to be interfered with. An example of such constant-current circuitry is shown in FIG. 1B, in which reference characters 7 and 7' indicate PNP transistors.
According to the knowledge of the inventor of the present invention, with the constant-current circuitry illustrated in FIG. 1B, no problems occur when a frequency of several tens to several hundreds KHz is used. When a frequency of more than several MHz is used, however, even with the above constant-current circuitry, the circuit block 1 and the circuit block 2 will interfere with each other. As a result of such interference, it is impossible to guarantee the accurate operations of the circuitry. Based on the result of numerous experiments and investigations performed by the inventor, it seems that the interference is ascribable to the parasitic capacity C.sub.BC between the PNP transistors 7, 7' and the collector/base. For a high frequency region of more than several MHz which is greatly susceptible to interference, it has therefore been proved that a technical means is required to solve such a problem.